The detection of shock and crack location provides important information to researchers or technicians concerned with underground nuclear and explosives testing, earthquake detection, and structural failure diagnostics. Electrical sensing systems are susceptible to electromagnetic interference, including lightning, and other atmospheric conditions which will tend to affect the accuracy of typical electronic measurement systems. Optical detection systems employing bulk optics, as opposed to optical fibers, often suffer from loss of alignment and cleanliness of their components, particularly in a field environment.
U.S. Pat. Nos. 5,107,129 (Lombrazo) and 5,142,141 (Talet et al.) disclose fibers that are broken as part of their detection process. Talet involves the detection of cracks while Lombrazo detects burn rate, both using optical fibers. The primary embodiments of Talet's multiple-loop arrangement is used. The breakage of one loop after another indicates the arrival of the burn front or the crack at the position of the loop. Both disclosures implicitly assume that each loop will be broken in an equivalent position. Although both disclosures differ in terms of proposed function, they are essentially structurally identical.
U.S. Pat. No. 4,843,234 (Berthold et al.) involves the measurement of the length of a single fiber using Optical Time Domain Reflectometry, which is a well known technique for determining the round trip travel time of a light pulse down the length of an optical fiber. The shortest length change detectable depends on the pulse repetition rate and the pulse length based on reflection.
Finally, U.S. Pat. No. 4,936,649 (Lymer et al.) mentions interdigitated optical fibers and "volume backscattering" as a means of determining the location of structural damage.
The present invention would provide a continuous fiber optic means of measuring shock location. Depending on the strength of the cable surrounding the fiber, this device could be designed with virtually any threshold crush pressure. One could also have considerable latitude in choosing the length over which this device is sensitive to shock pressure. The optical nature of the sensor causes it to be immune to electromagnetic interference and incapable of transmitting electrical signals that may contain sensitive information to the outside world. The optical nature of the sensor also reduces inaccuracies in the system that could be caused by various sources of electromagnetic interference. These features are in contrast to those of the leading existing devices, such as the SLIFER (Shorted Location Indication by Frequency of Electrical Resonance) and CORRTEX (Continuous Reflective Radius Time Experiment) coaxial cable type transducer devices whose outputs are discrete and electrical, and whose minimum crush strength is known to produce misleading measurements at low shock pressures. The present invention also overcomes the shortcomings of current fiber-optic devices that suffer from poor spatial resolution and bulkoptic devices that suffer from alignment problems. It is therefore desirable to fulfill the need for a shock and damage location detector that overcomes the shortcomings of the current art electrical and optical detectors.